This invention relates generally to an improved method for starting up and synchronizing a combined cycle turbine of the type having a gas turbine and steam turbine on a single shaft. More particularly, the invention relates to a unit startup program and a unit loading program for a combined cycle turbine, which is carried out by a unified control system. The method includes providing startup from standstill, firing the gas turbine, carrying out acceleration control, protecting the steam turbine against excessive heating, synchronizing the unit to the line and loading the combined cycle turbine in an optimum manner.
In some large combined cycle power plants the steam turbine and gas turbine are solidly coupled on a single shaft to drive a single electrical generator. The primary source of energy input to the rotating machine is the fuel which is burned in the gas turbine combustors. This shows up almost immediately as power delivered by the gas turbine. The waste heat from the gas turbine generates steam. This steam is utilized by a steam turbine as a secondary source of power input to the rotating train which is generated by a heat recovery steam generator (HRSG). While there is some time lag before heat from the gas turbine exhaust gas manifests itself as a power input source in the form of steam available at the turbine control valves, the control of the two sources of energy must be coordinated in order to properly control and protect the rotating machinery.
When synchronized with the electrical grid the speed of the machine is determined by the frequency of the grid. Of the total mechanical power produced from the fuel to drive the generator, approximately two-thirds is produced by the gas turbine and one-third by the steam turbine from the thermal energy recovered from the gas turbine exhaust. In most cases, all of the steam produced by the heat of the gas turbine exhaust is expanded through the steam turbine. In other cases, some of the steam is extracted from the power cycle for process uses. In the former case, the steady state control of electrical output, therefore, is achieved entirely by controlling fuel flow, with the steam control valve or valves maintained in the fully open position. When not synchronized, on the other hand, either fuel flow to the gas turbine, steam flow to the steam turbine, or both, must be controlled to control speed, and there is not always a direct relationship between the two.
During startup, before sufficient steam is generated from the heat recovery steam generator, under some conditions the control valves to the steam turbine may be closed. With no steam flow through the rotating turbine blades, excessive "windage" will cause the turbine to overheat. U.S. Pat. No. 4,519,207 to Okabe et al has suggested that an ancilliary steam source be provided to introduce flow through the steam turbine in a single shaft combined cycle to avoid overheating of steam turbine due to windage loss.
Elaborate startup programs and control systems have been developed for starting up steam turbines and gas turbines. Combined cycle units in a plant made up of several units, each consisting of gas turbine and steam turbine on a separate shaft have been suggested, as described in U.S. Pat. No. 4,532,761--Takaoka, issued Aug. 6, 1985. This combined station control, therefor, deals with multiple, separately controlled shafts. Steam turbines have different startup problems than gas turbines, and the control systems have developed separately for the two types of prime movers in order to address these problems. This invention relates to a single combined cycle unit and its unified control system.
A gas turbine is incapable of self starting from standstill. Torque from an external source is required for cranking to a speed at which ignition can occur and then to a higher speed at which operation becomes self sustaining and the gas turbine produces sufficient torque to accelerate to operating speed.
Large combined cycle steam and gas turbines on a single shaft require a very large cranking device for starting. The prior art has suggested a separate starting motor for the combined unit or, if the combined unit is driving a generator, using the generator as a motor to crank the combined unit.
A conventional gas turbine startup program controls the cranking device and the sequential operations involved with the startup. A typical program is as follows:
(1.) Beginning with the fuel stop valve closed, the cranking device accelerates the unit to 25-30% of rated speed and holds for several minutes to purge the exhaust system combustible gases.
(2.) Speed is reduced to 10-15% of rated for light off. The fuel stop valve is opened, a fixed fuel flow is admitted and ignition initiated.
(3.) After light off, fuel flow is reduced to warm up level for one minute.
(4.) The cranking device is then set for maximum torque and the fuel flow command is programmed to increase on a predetermined schedule.
An error signal is the difference between a reference or desired value of an operating condition and the actual measured value of the operating condition. The gas turbine control system utilizes several such error signals to develop several fuel command signals which are applied to a "minimum value gate". The small fuel flow command generated by the startup fuel schedule is selected by the minimum value gate unless temperature or other limitations have a smaller fuel command signal. As speed approaches the governor setpoint, the speed error requires the smallest fuel command and becomes the controlling signal. An integrated gas turbine control system providing for open loop programmed start-up control with a number of closed loop constraints simultaneously controlling the gas turbine in accordance with operating conditions such as temperature, speed and acceleration is described in U.S. Pat. No. 3,520,133 issued July 14, 1970 to Daniel Johnson and Arne Loft.
A steam turbine, on the other hand, is self-starting as soon as steam is admitted through the control valve, but due to need to allow temperatures to equalize in the rotor and shell, startup programs have been developed for starting and loading a steam turbine in accordance with allowable thermal stress in a controlled manner as disclosed in U.S. Pat. No. 3,561,216--Moore, issued Feb. 9, 1971. Combining acceleration and speed control through the use of a minimum value gate are shown in U.S. Pat. No. 3,340,883--Peternel, issued Sept. 12, 1967.
Unified control systems have been proposed for single shaft combined cycle plants with supplemental firing of fuel in the heat recovery steam generator which attempted to force a programmed load split between the gas turbine and the steam turbine, such a system being disclosed in U.S. Pat. No. 3,505,811 to F. A. Underwood issued Apr. 14, 1970. However, improved thermodynamic performance can be achieved by designing the system so that the steam valve operates in the full open position. In this way, the steam turbine accepts the total generation capacity of the steam generator over the entire load range without responding to small or slow speed variations which would require steam valve adjustment.
As load is increased on the gas turbine, more heat energy will flow with the exhaust gas to the HRSG where it will cause an increase in steam flow to the steam turbine. This will cause the steam pressure to rise so that the steam turbine will absorb this flow without any control action. A reduction in gas turbine load will, in similar manner, result in a reduced steam flow to the steam turbine. Thus, the steam turbine will follow the load changes on the gas turbine with some time delay.
While this provides optimum thermodynamic performance under steady state or slowly varying load changes, disturbances in steady or quasi-steady operation may occur. Two of these will be discussed in the following, (1) Proportional control, and (2) Power load unbalance control:
(1.) A gradual rise in shaft speed above rated speed will cause the gas turbine speed control to reduce fuel flow and hence power to the shaft in a proportional manner with speed rise. According to the present invention, as long as the shaft speed is below a preset value, the steam turbine will only respond by a reduced output as the steam flow from the HRSG is reduced.
A rise in combined shaft speed above the preset value will cause the steam valves to go closed in a manner proportional to the speed rise. This will reduce the steam flow to minimum flow level and hence shut off the steam flow as a contributor to excessive overspeed.
(2.) In the event of sudden loss of full electrical load, the above described proportional action may not occur fast enough to limit the speed rise of the unit to a value that will not cause the overspeed trip to activate, typically at 110% rated speed. Modern fossil fired steam turbines use a power-load unbalance system to control overspeed to a value below that of the setting of the overspeed trip. This permits the unit to experience a load rejection, yet remain running under speed control at or near synchronous speed. Thus, the unit can, if desired, continue to carry station auxiliary load and also be in a condition for prompt resynchronizing with the system. Such power load unbalance systems are shown in U.S. Pat. No. 3,198,954 in the name of M. A. Eggenberger et al issued Aug. 3, 1965 or in U.S. Pat. No. 3,601,617 to DeMello et al issued Aug. 24, 1971.
Accordingly, one object of the present invention is to provide an improved method for starting, synchronizing and loading a single shaft combined cycle turbine.
Another object of the invention is to provide an improved unified control system for coordinating controlled startup, synchronization and loading of a single shaft combined cycle plant, including transfer of control between steam turbine and gas turbine.